Stored program controlled communication switching systems comprise some form of intelligence which controls switching functions in response to a program stored in memory. Historically, such systems included a single processing entity for the control of the entire system. As technology and system design evolved, it was found desirable to separate certain routine functions from the main processing entity to save its processing time for more complex system functions and decisions. Today, systems are being designed which also separate some of the more complex system functions and decisions into several intelligent processors.
One type of modern time-division switching system comprises a plurality of time-slot interchange units each including a processor, which is selectively interconnected by a time multiplexed switch, sometimes called a time-shared space division switch. Such a switching system also includes a central control arrangement including a processor which determines the pattern of interconnections between the plurality of time-slot interchange units. In order to coordinate the actions of the time-slot interchange unit, the time multiplexed switch, and the central control arrangement, control messages must be exchanged. Since each control message occupies some portion of the interprocessor communication media, too many messages can reduce system real time response by causing delays in message transmission. Similarly, since each message requires processor real time, the individual processors can be kept from needed tasks while receiving and transmitting messages. According1y, it is important to minimize the number of control messages exchanged in the system. It is also important, however, that each unit in a distributed processor system have enough information to enable it to properly perform its tasks. This is especially true in distributed telecommunications systems where the integrity of connections must be maintained. Thus, the portions of a distributed communications systems must be designed to operate accurately with a minimum number of required control messages.
An example of the conflict between proper operation and a potentially large number of control messages occurs with communications between the time multiplexed switch and the central control arrangement. The central control and time multiplexed switch must cooperate to complete connections for virtually all calls traversing the switching system. In prior art systems, the central control arrangement, when requested, transmits a message to the time multiplexed switch defining a connection between time-slot interchange units. Then, when the connection is no longer needed (a subscriber goes on-hook), a second message is transmitted directing the time multiplexed switch to clear the connection between the time-slot interchange units. If unused paths through the time multiplexed switch are not cleared, a given input port may eventually be simultaneously connected to a number of output ports and when the given input port is used to complete another connection, signals arriving at that input port may be broadcast to a number of output ports. As discussed above, prior systems transmit two control messages for each time multiplexed switch connection. The first message is used to set up the connection. The second message is then used to remove it. In accordance with the present invention, multiple output port connections to a given input port are substantially avoided without the necessity of disconnect orders, thus reducing substantially the number of control messages exchanged between the central control arrangement and the time multiplexed switch in a time-division switching system. Similar advantages can be obtained in the interactions between time-slot interchange units and the central control.